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When you crack open an egg, you split the shell and break it into smaller pieces. This reveals the desired inner white and yolk. Chemical cracking is a bit like that.
Cracking is the process of breaking down large molecules into smaller, more useful ones.
The term cracking is most commonly used to describe the breakdown of long-chain hydrocarbons into shorter-chain alkanes and alkenes, which is what we’ll look at in this article.
Crude oil, as explored in Fractional Distillation, produces many different hydrocarbon fractions. These all have different purposes and demands. For example, there is a huge demand for naphtha, a fraction with carbon chain lengths between about 6 and 12, as we use it for petrol and in the chemical industry. On the other hand, there is a lower demand for longer-chain hydrocarbons. They are not that useful and so have low economic value. This is a problem, as typical crude oil from the North Sea contains only 10 percent naphtha, but 66 percent gas oil and fuel oil fractions, which have chains over 16 carbon atoms long. We end up with a lot of spare longer-chain hydrocarbons, with not much to do with them!
The composition of typical North Sea crude oil. Anna Brewer, StudySmarter Originals
To make longer-chain fractions more economically valuable, we can crack them to produce shorter-chain molecules. This has the benefit of producing not only shorter alkanes, but also alkenes, which we’ll introduce below. These hydrocarbons are in much higher demand and are a lot more useful to us than longer-chain hydrocarbons, making cracking such an important reaction. But what exactly are these products used for?
You’ll probably find examples of short-chain alkanes such as butane, 
, everywhere in your local environment. Butane is commonly used in cigarette lighters, in aerosols, as a food additive, in fridges, and even as fuel for your car.
Butane. commons.wikimedia.org
Alkenes are unsaturated hydrocarbons.
An unsaturated molecule contains at least one C=C double bond.
Alkenes are more reactive than alkanes and are used as a chemical feedstock, which means that they supply industries with the starting materials used to make further products. Reacting multiple alkene molecules together forms plastic polymers, such as those used in plastic bags, bottles, and nylon clothing. We also use them as a base for making alcohols, paints, and medicines.
The simplest alkene, known as ethene. commons.wikimedia.org
To find out more about these hydrocarbons, check out Alkenes.
There are two different types of cracking we commonly use to split hydrocarbons. These are known as thermal cracking and catalytic cracking. Because hydrocarbons like alkanes are relatively unreactive due to their strong, non-polar C-C and C-H bonds, both types of cracking require certain harsh conditions to break them down.
Thermal cracking involves putting the alkanes under extreme heat and pressure for a brief period of time, usually only one second. We typically use a temperature of 700-1200K and a pressure of 7000 kPa.
The alkane splits homolytically, meaning one electron from the bonded pair goes to each of the new molecules formed. This forms two free radicals.
A free radical is an extremely reactive molecule with an unpaired outer shell electron.
Free radicals react further to produce many different hydrocarbons, but especially alkenes. However, maintaining such extreme conditions requires a lot of fuel. Therefore thermal cracking has a large economic and environmental footprint.
A catalyst is a substance that increases the rate of reaction by lowering the activation energy needed for a reaction to occur.
Because catalysts increase the rate of reaction, it shouldn’t surprise you that catalytic cracking uses a lower temperature and vastly reduced pressure compared to thermal cracking. For more information, see Increasing Rates. Catalytic cracking takes place at 700K and a pressure only slightly above atmospheric pressure, but uses a zeolite catalyst. This is a complex lattice made from aluminium, silicon, and oxygen, with a honeycomb structure to increase its surface area. Unfortunately, larger alkanes can’t be cracked in this way. They are too large to fit in the catalyst.
Catalytic cracking produces a high proportion of cyclic and branched alkanes, and aromatic compounds such as benzene. It also has much lower fuel requirements than thermal cracking.
An aromatic compound contains the benzene ring.
The following table compares the two common types of cracking:
A table comparing thermal and catalytic cracking. Anna Brewer, StudySmarter Originals
Cracking is a largely random process. It is impossible to predict exactly which molecules will be produced. This means there are multiple different equations and potential products for each reaction. The important thing to remember is that the total number of carbons and hydrogens on each side of the equation must be equal. Let’s have a go.
Decane, 
, can be cracked to produce octane, 
and one other molecule. Write a balanced equation for the reaction.
Let's write out our equation. So far it looks like this:
Here, x and y represent unknown quantities of carbon and hydrogen atoms respectively. However, we can work these values out.
We know there have to be ten carbons on the right-hand side of the equation, because there are ten on the left. We already have eight, and so 10 - 8 = 2 carbons remaining. Therefore, x = 2. Likewise, we have 22 - 18 = 4 hydrogen atoms remaining. Thus y = 4. Our other product is ethene, 
.
Here is another example:
One molecule of alkane X can be cracked to produce one molecule of heptane and two molecules of propene. Deduce the formula of X.
Again, our equation looks like this:
There are 7 + (2x3) = 13 carbons on the right-hand side of the equation. Likewise, there are 16 + (2x6) = 28 hydrogens. As there is only one molecule reacting, the alkane on the left must be 
.
Cracking is the process of breaking down-longer chain fractions from the fractional distillation of crude oil into shorter lengths.
Longer-chain hydrocarbons have a low demand, so are cracked to produce more economically valuable shorter-chain hydrocarbons.
Catalytic cracking and thermal cracking.
Thermal cracking requires a temperature of 700-1200K and a pressure of 7000 kPa. Catalytic cracking requires a zeolite catalyst, a temperature of 700K, and a slightly raised pressure.
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